Many systems exist which trigger an automatic response from a device. These systems allow user interaction, based upon, either the distance of the user to the device, or a predefined position of the user with respect to the device. This may, for example, involve activating a screen containing a welcome message, turning on a light, playing a sound, etc. When the user leaves the area of the device, the system is reset to a default state (for example the screen is turned off, lights are turned off, etc.).
Specific applications are found extensively in automatic lighting control. A presence detector is capable of detecting if a user enters a predefined zone such as a room. Once such an event is detected, a particular lighting condition is applied to the zone. This may include functional light for creating a particular atmosphere.
Another specific application relates to activation of a user interface such as a computer system, television, media players, hi-fi, etc. In this case, the distance of the user with respect to the user interface is detected and once the distance is less than a certain threshold, the user interface is activated and may show a welcome message on screen, and optionally gesture control is activated. Once the user is at a distance outside the range of the threshold, the user interface is turned off.
In the above existing systems, one approach to operate the system is to select a desired distance range/area inside which the system should be activated; if the user steps inside this zone, the system is activated; and if the user steps out of this zone, the system is deactivated.
While this technique is intuitive, it has disadvantages. If a user enters a zone, but does not proceed further (i.e. the user remains very close to the boundary of the zone), a very small movement away from the boundary will trigger the system to go back to the initial/default state. From that point, a very small movement toward the zone boundary will trigger yet another change of state, and so forth making the behaviour of the system very unstable as it flickers on and off etc. This effect is illustrated in FIGS. 1a-d and 2a-d. 
FIGS. 1a-1d illustrates the effect of large movements of the user with respect to an activation zone of an existing system. In FIG. 1a, a device 101 comprises an activation zone 103. A user 105 is located outside of the activation zone 103 and consequently the device 101 is turned off. As the user 105 enters the activation zone 103, crosses the boundary of the activation zone 103, as shown in FIG. 1b, the device 101 is activated. As the user 105 continues to move toward the device 101 and completely enters the activation zone 103, as shown in FIG. 1c, the device 101 remains activated. When the user 105 leaves the activation area 103, as shown in FIG. 1d, the device 101 is deactivated. Therefore, the system behaves as expected such that the device 101 is activated and remains activated whilst the user 105 is within the activation zone 103 and is deactivated and remains deactivated whilst the user 105 is outside the activation zone 103.
However, in the event of smaller movements of the user problems arise in such systems as illustrated in FIGS. 2a-d. Similar to FIG. 1a, FIG. 2a shows the user 105 located outside the activation zone 103 of the device 101. The device 101 is deactivated, turned off. As the user 105 approaches the activation zone 103, as shown in FIG. 2b, the device 101 is activated. If the user 105 then remains in the area of the boundary of the activation zone 103, a small movement of the user 105 in this area may cause the user 105 to be detected as outside the activation 103, as shown in FIG. 2c. Consequently, the device 101 is deactivated. These small movements may equally detect that the user 105 is within the activation zone 103, as shown in FIG. 2d, which then activates the device 101. As a result the small movements of the user 105 in the area of the boundary of the activation zone 103, causes flickering of the device 101 between on and off which is annoying and confusing to the user.
This is partly overcome by use of a timer, so that, only after a predetermined time interval, the switch to the default state is triggered. Therefore, small rapid movements back and forth will not trigger the device to turn on and off with each movement. However, even in this case, a problem remains: if the user is close to the boundary and moves away a very small amount, the system will reset itself after a delay, making the interaction non intuitive. If the timer is set too long, the interaction state of (lighting or user interface device) will remain on for too long, making it not practical and inefficient.